1. Field of the Invention
The present invention relates to a process for the hydroconversion of heavy hydrocarbonaceous fractions of petroleum. In particular, it relates to a close-coupled two-stage; thermal-catalytic, catalytic-hydrotreatment process for converting petroleum heavy oils that provides improved effectiveness for high conversion and control of condensation reactions to produce stable high-quality products.
2. Background
Increasingly, petroleum refiners find a need to make use of heavier or poorer quality crude feedstocks in their processing. As that need increases, the need also grows to process the fractions of those poorer feedstock's boiling at elevated temperatures, particularly those temperatures above 1000° F. High conversions to stable, quality products are desirable in order to avoid producing significant quantities of low value fuel oil. Delayed Coking, the refiner's traditional solution for converting heavy oils to liquid products, has become less attractive because of the low conversion to liquid products and the relatively low value of the coke by-product. Higher liquid conversions can be achieved with conventional ebullated bed technologies. But these technologies, even with enhancements such as solvent de-asphalting, suffer limitations due to the instability of the fuel oil product and refractive nature of the products—making further upgrading difficult.
Severe conditions are required in order to achieve high conversions which, while producing desirable lighter fractions, can also produce thermally cracked fragments and unstable asphaltenes that form mesophase masses. Unless controlled, the cracked fragments undergo condensation reactions to undesirable polycyclic molecules which tend to be unstable and difficult to process into desirable products. Along with the mesophase masses, they can also lead to coke formation.
The key to high conversion and product quality is the management of the asphaltenes which are produced at severe operating conditions. Current approaches have focused on slurry reactor technology utilizing sophisticated dispersed catalyst systems, in some cases employing molybdenum. These technologies tend to have high investment and operating costs and, in some cases, product quality remains an issue. Many of these processes also have difficulties if the metals content of the feedstock is high.
Various processes for the conversion of heavy hydrocarbonaceous fractions, particularly, multi-stage conversion processes include those described in U.S. Pat. No. 4,761,220, Beret, et al.; U.S. Pat. No. 4,564,439, Kuehler, et al.; U.S. Pat. No. 4,330,393, Rosenthal, et al.; U.S. Pat. No. 4,422,922, Rosenthal, et al.; U.S. Pat. No. 4,354,920, Rosenthal, et al; U.S. Pat. No. 4,391,699, Rosenthal, et al.